Controlled plasma arc cutting

ABSTRACT

A process and apparatus for cutting a material using a plasma are jet provides improved uniformity along the length of cut, despite variations in cutting speed. This is achieved by pulsing the arc current and dynamically varying the pulsing. By this means the momentum of the plasma arc jet can be maintained substantially constant whilst the amount of energy delivered by the plasma arc jet is controllably varied. The pulsing can be dynamically varied in dependence on one or more of the cutting speed, the angle of ejection of a stream of molten material from the cut, the size of the droplets of the ejected material, the intensity of spectral pattern of light emitted from the plasma arc jet and material interface, and the arc voltage. The pulses can be varied by varying one or more of the pulsing frequency, the pulse duty, upper value of the pulse current, and depth of the pulses.

TECHNICAL FIELD

This invention relates to a plasma arc cutting process and apparatus. Inparticular, the invention is concerned with controlling the electric arcin plasma arc cutting applications, such as in the metal fabricationindustry.

BACKGROUND

In plasma arc cutting technology, the quality of a cut can be describedin terms of the dimensional accuracy of the cut parts, cut angle (degreeof squareness of the cut face in the direction normal to the cut), theamount of dross on the bottom of the workpiece (which will usually bemetal plate), amount of spatter on the top of the workpiece andsharpness of the top and bottom edges of the cut part. Cut quality isdetermined, in particular, by the effectiveness of metal melting andremoval from the workpiece, which depends on factors including thethermal energy delivered to the workpiece and on the momentum of theplasma jet.

The thermal energy delivered to the workpiece depends on the electricalenergy of the plasma arc and on the efficiency of energy transfer to theworkpiece. If it is assumed that the energy transfer efficiency issubstantially constant, then the electrical energy of the plasma arc canbe taken as a measure of the thermal energy delivered to the workpiece,which is the approach adopted in the following description.

The amount of electrical energy per unit length of cut is a significantprocess variable affecting the material melting process. This processvariable is determined by the cutting speed, arc voltage, arc currentand pressure of the plasma forming gas. Conventional process control forplasma arc cutting relies on regulation of the cutting speed, arcvoltage, arc current and plasma gas pressure around constant (optimal)set points which are chosen to ensure the best cut quality for a givenplate. In general, the optimal cutting speed cannot be maintained at alltimes, for example, for a plasma arc cutting operation that isintegrated with a manufacturing process such as welding, as incontinuous pipe making, the optimal welding speed may determine the useof a non-optimal cutting speed. Also the optimal cutting speed cannot bemaintained during the cutting of complex parts using profiling machinesbecause of the finite acceleration capabilities of these machines. Thatis, the deceleration along the x-axis and acceleration along the y-axisof such a machine during the traversal of a 90° corner results in adecrease of the cutting speed near the corner.

The effect of variations in the cutting speed on the amount of energyper unit length of a cut is twofold. First, the amount of energy perunit length of cut increase with decreasing cutting speed for constantarc voltage, arc current and pressure in the nozzle chamber. Second, thearc voltage increases with decreasing cutting speed thus furthercontributing to the increase in the amount of energy per unit length ofcut. Such an increase in the arc voltage is due to an effective increasein the length of the arc caused by the movement of the arc anode rootdown the cutting front at reduced cutting speed.

The increase in the amount of energy per unit length of cut when thecutting speed reduces results in an excessive amount of molten metalwhich cannot be completely removed by the momentum of the plasma jet.Further, at low cutting speeds the shape of the cut front changesresulting in a change in the direction of ejection of molten metal. Thisleads to dross formation and possibly to corner undercut. Dross is oftenformed well beyond the deceleration-acceleration region of a cornerwhich is due to the shape of the cut front. Since the cut front dependsupon the diffusion of heat through the plate, there is a time dependentmechanism associated with dross formation initiated in the vicinity ofthe profile corner. This means that a significant part of the profilemay be affected by dross formed at the bottom of the plate.

In the prior art the amount of energy per unit length of cut has beencontrolled by varying the torch-to-workpiece distance. However theamount of variation in this distance that is available and its effect onthe amount of energy is generally insufficient to eliminate drossformation in the vicinity of profile corners. The amount of energy perunit length of cut may also be controlled by varying the arc current inresponse to changes in the cutting speed, for example, by decreasing thearc current while decreasing cutting speed. This type of control of theamount of energy per unit length of cut may ensure effective metalmelting, however the cut quality also depends on the effectiveness ofmetal removal from the workpiece, which in turn depends on the momentumof the plasma jet.

It can be shown that the momentum of a plasma arc jet emanating from thenozzle of a plasma arc cutting torch is approximately proportional tothe gas pressure in the nozzle chamber and that there is a strongrelationship between this pressure and arc current. Thus the pressure inthe nozzle chamber, and therefore the plasma jet momentum, varies withcurrent. The effect of this is that although the energy per unit lengthof cut could be controlled effectively by varying the arc current, thisis at the expense of the plasma jet momentum. That is, a decrease in arccurrent results in a decrease in the momentum of the plasma jet and thisreduces the effectiveness of metal removal by the plasma jet. Thuseffective metal removal and therefore a high quality of cut around aprofile cannot be maintained.

Japanese Patent No. 1884596 (Kokai 61-262464) by S Hagihara et al,discloses pulsing of the arc current to reduce the amount of dross andto enable high speed cutting. Soviet patent document SU-1632670-A, alsodiscloses pulsing of the arc current to increase the cutting speed.However in both of these disclosures the arc pulsing parameters arefixed during cutting. Japanese Kokoku 44-29967 also discloses pulsing ofthe arc current, but with a cyclic variation in the amplitude of thepulses to uniformly distribute the heat of the plasma arc down the depthof the cutting groove to reduce narrowing and irregularities in thisgroove.

Although the above disclosures for pulsing the arc current give improvedcut quality, the uniformity over the length of a cut, particularly whenproducing parts having a complex profile, is variable. Also in the aboveJapanese and Soviet patent documents, the current pulsing is “upwards”in the sense that the torch is operated at or near its DC rating andcurrent pulses of increasing amplitude are imposed on this DC currentsuch that generally the current rating of the nozzle is momentarily(i.e. for the duration of each pulse or over a lesser period) andrepeatedly exceeded. This can lead to problems such as double arcing andthus a shortened lifetime for the torch consumables.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a plasma arc cuttingprocess and apparatus giving improved uniformity along the length of acut.

According to the invention there is provided a process of cutting amaterial using a plasma arc jet, wherein a current supplied to a torchfor generating the plasma arc is pulsed and the pulses are varied duringthe cutting process, and wherein the pulse variations are such that themomentum of the plasma arc jet is maintained substantially constantwhile the electrical energy of the arc is varied.

The inventors have discovered that the electrical energy of the plasmaarc (and thus the thermal energy delivered to the workpiece) can bevaried to maintain it substantially at an optimum level throughout thelength of a cut whilst maintaining the momentum of the arc jet at alevel that ensures effective removal of the material, that is, theinventors have effectively decoupled the jet momentum from the arccurrent such that their combined effect gives an improved result, namelycuts of enhanced uniformity. This is achieved by pulsing the arc currentand varying the pulse parameters.

Preferably the arc current is pulsed “downwardly” in the sense that theupper current value for the pulses is typically the current rating ofthe nozzle. This feature is believed to contribute to an improvedconsistency of operation over prior art torches over a number ofoperations, that is, the cut quality and its repeatability are enhanced.

The pulse parameters that may be varied include the pulsing frequency,pulse duty, upper current value and depth of pulsing. “Pulse duty” asused herein means the ratio of the time during which the current is atits lower value (the arc current pulse duration) and of the sum of thetime during which the current is at its lower and upper values (periodof the arc current pulsing). The “depth of pulsing” is defined as:

Depth of pulsing=(upper current value−lower current value)/upper currentvalue.

The momentum of the jet may be maintained substantially constant bycontrolling the pulsing frequency. As is described above, the momentumof the jet is approximately proportional to the pressure in the nozzlechamber, that is, the momentum is determined by the nozzle chamberpressure. Thus, if fluctuations in this pressure could be avoided whilstpulsing the arc current, the momentum of the jet ought to remainunaffected. The time constant of the pressure response to a step changein the arc current depends, in particular, on the volume of the nozzlechamber and is usually in the order of only a few milliseconds. Thus, ifthe period of the arc current pulsing is made sufficiently short,significant fluctuations of the pressure in the nozzle chamber can beavoided. The inventors have determined that the pressure fluctuations inthe nozzle chamber are insignificant when the period of the arc currentpulsing is of the order of the time constant of the pressure response toa step change in the arc current. A suitable range of the pulsingfrequency is about 150 Hz to 600 Hz.

The invention also provides plasma arc cutting apparatus including, aplasma arc torch, a power supply for supplying a current to the torchfor generating a plasma arc, and means for pulsing the current to thetorch, wherein the pulsing means is operable to variably pulse the arccurrent such that the momentum of the plasma arc jet is maintainedsubstantially constant whilst the electrical energy of the arc isvaried.

Preferably in a process according to the invention, the electricalenergy of the arc is controllably varied in dependence on a cuttingprocess variable. Thus, apparatus according to the invention preferablyalso includes means for measuring a cutting process variable to supply asignal to the pulsing means to variably pulse the arc current independence on the cutting process variable. Preferably this variable isthe cutting speed, however other variables that may be used include theangle of a stream of molten material ejected from the cut (kerf), thesize of droplets of molten material ejected from a workpiece, theintensity or spectral pattern of light emitted from the plasma arc jetand material interface, or the arc voltage.

Controlling the arc current pulsing in an on-line manner depending onsome cutting process variables allows a high and uniform cut quality tobe obtained for a range of cutting speeds. Optimal control of the arccurrent pulsing involves on-line control of the pulsing frequency, pulseduty, upper current value and depth of pulsing. However the inventionincludes simplified control strategies involving on-line control of someof the parameters of the arc current pulsing while the remainingparameters are fixed. For example, the pulse duty, upper current valueand depth of pulsing may be controlled on-line while the pulsingfrequency is fixed. This allows simplification of the control apparatus.

The relationship between the speed of cutting and amount of energy perunit length of cut supplied by the arc, and how this can cause anexcessive amount of molten material to be produced when the cuttingspeed slows, has been described above. Thus, according to the strategyof the present invention, as the cutting speed slows, the arc currentpulses are varied in a dependent manner to maintain the momentum of thearc and to reduce the electrical energy of the arc and thus the amountof molten material. The other cutting process variables that are givenabove may be similarly used: thus an excessive lead (or lag) angle ofthe molten stream of material ejected from the kerf indicates that anexcessive (or insufficient) amount of energy per unit length of cut isbeing delivered to the workpiece. Likewise, large droplets of moltenmaterial indicate the delivery of excessive amount of energy per unitlength of cut.

For a better understanding of the invention and to show how it may becarried into effect, embodiments thereof will now be described, by wayof non-limiting example only, with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a plasma arc torch to which theinvention is applicable.

FIG. 2 schematically illustrates a profiling machine incorporating thetorch of FIG. 1.

FIG. 3 shows how cutting speed varies during traversal of a corner usinga machine as in FIG. 2.

FIG. 4 shows the variation in electrical energy per unit length of cutas a result of the cutting speed variation shown in FIG. 3.

FIG. 5 shows the relationship between the pressure in the nozzle chamberand the arc current.

FIGS. 6 to 8 show the relationship between dross height and pulse dutyfor a range of cutting speeds.

FIG. 9 shows the arc voltage, arc current and cutting speed during thetraversal of a corner while cutting according to the process of theinvention.

FIG. 10 shows the attainment of a substantially constant nozzle pressure(and hence plasma arc jet momentum) during the traversal of a cornerwhile cutting according to the process of the invention).

FIG. 11 shows a sample cut according to the prior art.

FIG. 12 shows a sample cut according to the process of the invention.

FIG. 13 shows the arc voltage, arc current and cutting speed during thetraversal of a corner while cutting according to the process of theinvention-using a different control strategy to that illustrated by FIG.9.

FIG. 14 is a functional diagram for apparatus according to theinvention.

FIG. 15 is an electrical diagram for apparatus according to theinvention, and

FIG. 16 is a graph for explaining the operation of FIG. 15.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A cross-section of a typical transferred arc, dual flow plasma arc torch20 is shown in FIG. 1. The torch comprises a body (not shown) whichincludes an electrode 22 (typically the cathode) centred above a bore 24in a constricting nozzle 26. Both the electrode 22 and nozzle 26 may bewater cooled to reduce their erosion (for example, coolant flow for theelectrode 22 is shown by arrows 28). A suitable plasma forming gas 30(e.g. air, oxygen, nitrogen or a mixture of argon and hydrogen) flowsunder pressure around the electrode 22 and through the nozzle bore 24towards a workpiece 32. The plasma forming gas 30 may pass through aswirl ring 34 that improves arc stability and squareness of the cut onthe part side of the kerf. An arc 36 from the torch 20 is constricted bythe flow of gas 30 through the nozzle 26, further constriction can beachieved by applying a shield gas 38, or by water as in a waterinjection torch or by a secondary nozzle as used in high precisioncutting torches. The nozzle 26, a shield 40 and electrode 22 are usuallymade of copper. An electron emitting element 42 inserted in the tip ofelectrode 22 is made of hafnium, zirconium or tungsten depending on theplasma forming gas.

Molten material ejected from a workpiece 32 as it is cut by a plasma arcjet 36 is shown by reference 37.

A diagram of a typical mechanised plasma arc cutting system with aplanar profiling machine is shown in FIG. 2. This consists of a cuttingtorch 20 mounted on a carriage 44 movable along x and y axes under thecontrol, for example, of a CNC controller 46. The system also includes apower supply 48, arc igniter 50 and requires plasma forming and shieldgases 52 as well as torch coolant 54. In the case of a transferred arctorch 20, one terminal 56 of the power supply 48 (typically negativepolarity) is connected to the electrode (cathode) 22 of the torch 20 andthe other terminal 58 to the workpiece (anode) 32 which is mounted on acutting table 60. The arc igniter 50, which is a high-frequency,high-voltage generator, is used to establish a pilot arc between theelectrode 22 and nozzle 26 of torch 20. Subsequently, under theinfluence of a strong gas flow, the arc transfers to the workpiece 32.

The invention includes use of a plasma arc cutting torch 20 that can behand-held, or mounted on a profiling machine (as in FIG. 2), on a 3-axisgantry or on an articulated robotic manipulator.

The results presented in this description were obtained using aHypertherm MAX200 plasma arc cutting system. This system includes a 200A chopper-based power supply. Either air or oxygen was used as theplasma forming gas. Air was used as the shield gas. The machine used toguide the torch above the workpiece was a Farley Cutting System WizardII. This machine comprises gantry elements which translate in orthogonaldirections.

An optimal cutting speed cannot be maintained during cutting of complexparts using a profiling machine as in FIG. 2 because of the finiteacceleration capability of these machines, for example, the decelerationof the x-axis and acceleration of the y-axis during traversal of a 90°corner results in a decrease of the cutting speed near the corner, asshown in FIG. 3.

The overall effect of this change in cutting speed on the amount ofenergy per unit length of cut is illustrated in FIG. 4 depicting thevariation of the electrical energy during the above-mentioned cutting ofa corner (100 A current, 3 m/min optimal cutting speed). The increase ofthe energy near the corner of the profile results in excessive amount ofmolten metal which cannot by completely removed by the momentum of theplasma jet. This leads to dross formation and possibly to cornerundercut. The dross is often formed well beyond the cornerdeceleration-acceleration region, that is, there is a memory effectassociated with dross formation initiated in the vicinity of the profilecorner. This means that a significant part of the profile may beaffected by dross formed at the bottom of the plate.

The amount of energy per unit length of cut may be controller by varyingthe arc current in response to changes in the cutting speed, forexample, by decreasing the arc current with decreasing cutting speed.This type of control of the amount of energy per unit length of cut mayensure effective metal melting. However, the cut quality depends also onthe effectiveness of metal removal from the workpiece. Thiseffectiveness of metal removal depends on the momentum of the plasma jetwhich is determined by the arc current.

The force exerted by the plasma jet on the molten material at the cut isa function of the momentum of the plasma jet which is proportional tothe product of the mass density of the plasma and the square of itsvelocity. The nozzle of the plasma torch 20 has a straight bore andhence at the operating mass flow rates used for cutting, the velocity ofthe plasma at the nozzle exit 24 is sonic, as evidences by opticalobservations, which show a supersonic jet 36 emanating from the nozzle.Owing to the choking or sonic flow at the nozzle exit 24, the gasdynamic behaviour of the plasma described in terms of mass flow and jetmomentum are primarily determined by the conditions in the nozzlechamber located upstream of the nozzle exit. Since the momentum of mostgases used for plasma arc cutting is proportional to pressure over awide range of temperature of 1000-30000 K, the momentum of the plasmajet 36 and the consequent force exerted by the jet on the moltenmaterial are approximately proportional to the pressure in the nozzlechamber.

The presence of a high-temperature, and hence low-density, arc plasma inthe nozzle restricts the flow of gas through the nozzle. If the rate ofmass flow through the nozzle exit is maintained approximately constant,the pressure in the nozzle chamber rises to overcome the restrictiveeffect of the plasma and maintain the mass flow rate.

FIG. 5 (which is for a 100 A nozzle) shows that there is a strongrelationship between the pressure in the nozzle chamber and arc current.Thus, the pressure in the nozzle chamber, and therefore the plasma jetmomentum, varies with current and a strategy of controlling the amountof energy per unit length of cut by varying the arc current depending onthe cutting speed would result in the momentum of the plasma jetdecreasing during cutting of the profile corners. This means thateffective metal removal and therefore high cut quality would not bemaintained around the profile.

The process of the invention maintains the balance between the momentumof the plasma jet and the amount of energy per unit length of cut neededfor high cut quality. The process relies on pulsing of the arc currentdown from an upper current value (typically the value of the currentrating of the nozzle) for a short period of time in a repetitive mannerat a sufficiently high frequency. This approach facilitates dynamiccontrol of the amount of energy per unit length of cut without asignificant decrease in the momentum of the plasma jet.

FIGS. 6 to 8 illustrate that a simplified control strategy according tothe invention reduces the amount of dross. Thus FIGS. 6 to 8 show thedependency of the average dross height on the pulse duty and on thecutting speed for a fixed pulsing frequency, fixed upper current valueand fixed (50%) depth of current pulsing. FIG. 6 is for 6 mm mild steelplate cut at 100 A upper current value, 50% depth of pulsing, 240 Hzpulsing frequency, 100 A nozzle. FIG. 7 is for 6 mm mild steel plate cutat 200 A upper current value, 50% depth of pulsing, 200 Hz pulsingfrequency, 200 A nozzle. FIG. 8 is for 12 mm mild steel plate cut at 200A upper current value, 50% depth of pulsing, 150 Hz pulsing frequency,200 A nozzle. These Figs show that the optimal pulse duty increases withdecreasing cutting speed. Hence, a reduction of the amount of dross canbe obtained by using the following control law relating the pulse dutyto the cutting speed for the fixed pulsing frequency, upper currentvalue and depth of pulsing:

Pulse duty(t)=Nominal duty+Sensitivity×(Optimal cutting speed−Cuttingspeed(t))

Nominal duty is the pulse duty at optimal cutting speed and can be setat 0% for conventional DC cutting (i.e. no pulsing). A small value ofthe Nominal duty of, say, 5% may be used as well since the arc currentpulsing was found to have a desirable effect on cut quality even atoptimal speeds. The Sensitivity is defined as the required change in thepulse duty per unit change of the cutting speed. The value ofSensitivity can be selected experimentally. The value of 8% per m/minwas found to yield a significant dross reduction in the vicinity ofprofile corners for 6 mm and 12 mm mild steel plate cut at upper currentvalue 100 A and 200 A for 100 A and 200 A nozzles, respectively. The arcvoltage, current and cutting speed are shown in FIG. 9 during traversinga corner while cutting 6 mm mild steel plate at 100 A upper currentvalue, 50% depth of pulsing, 240 Hz pulsing frequency, 5% nominal pulseduty, 3 m/min optimal cutting speed and 8% per m/min sensitivity. Fromthe arc current waveform it can be seen there is an increase in thepulse duty with the decreasing cutting speed. The corresponding pressurein the nozzle chamber and the cutting speed are depicted in FIG. 10(which shows the plasma forming gas pressure in the nozzle chamber andthe cutting speed during traversing a corner while cutting 6 mm mildsteel plate at 100 A upper current value, 50% depth of pulsing, 240 Hzpulsing frequency, 5% nominal pulse duty, 3 m/min optimal cutting speedand 8% per m/min sensitivity) from which it is evident that the nozzlepressure, and therefore the momentum of the jet, is almost constant.Samples of parts cut under DC conditions (no pulsing) and under pulsedconditions as in FIGS. 9 and 10 are shown in FIGS. 11 and 12respectively. The sample workpiece 32 shown by FIG. 12 has very littledross 80 compared to the dross 80′ on the sample workpiece 32′ of FIG.11.

The peak in the energy per unit length of cut occurring in the vicinityof the corner under DC conditions (c.f. FIG. 4) is decreased by 20%under the above pulsing conditions. Furthermore, the pressure in thenozzle chamber decreases only by 2% during corner traversal. If theSensitivity were set to 30% per m/min, then the peak in the energy perunit length of cut would decrease by almost 40% whereas the pressurewould decrease only by 6%. It is to be understood that this order ofvariation in the pressure is encompassed in the definition that the arcmomentum is maintained “substantially constant”. These results show thatthe energy per unit length of cut can be controlled by the arc currentpulsing without affecting the nozzle pressure significantly. Thus,control of the energy per unit length of cut is effectively decoupledfrom control of the pressure in the nozzle chamber and therefore fromthe plasma jet momentum.

A more complex arc current pulsing algorithm may involve adjustment ofthe upper current value and of the depth of pulsing in response tochanges in the cutting speed. For example, the upper current value maybe controlled according to the following control law:

Upper current value(t)=Nominal upper current value−Sensitivityupper×(Optimal cutting speed−Cutting speed (t))

Furthermore, the depth of pulsing can be controlled indirectly bycombining the above control law for the upper current value with thefollowing control strategy for the lower current value:

Lower current value (t)=Nominal lower current value−Sensitivitylower×(optimal cutting speed−Cutting speed(t))

The Sensitivity upper (Sensitivity lower) is defined as the requiredchange in the upper (lower) current value per unit change of the cuttingspeed. Thus, the amount of energy per unit length of cut may becontrolled by adjusting the pulse duty and the upper and lower currentvalues. For example, the arc voltage, current and cutting speed areshown in FIG. 13 during traversing a corner while cutting 6 mm mildsteel plate with 100 A nozzle and at 3 m/min optimal cutting speed and240 Hz pulsing frequency with the following parameters: Sensitivity=8%m/min, Nominal pulse duty=5%, Nominal upper current value=100 A,Sensitivity upper=17 A per m/min, Nominal lower current value=60 A,Sensitivity lower=4 A per m/min.

A functional diagram for apparatus according to the invention is shownin FIG. 14. The apparatus comprises a Current Pulsing Controller 62, aCurrent Pulsing Module integrated with a power supply 64 and a plasmaarc cutting torch 20.

A current Pulsing Module 66 (see FIG. 15) of the functional block 64 ofFIG. 14 implements a non-dissipative current diversion technique topulse the arc current. In this embodiment two chopper modules in theHypertherm MAX200 power supply are used as two constant current sources68, 70. The Current Pulsing Module 66 diverts current from one of theconstant current sources 68 or 70 leaving only the other constantcurrent source driving the plasma. The principle of the Current PulsingModule 66 operation is depicted in FIG. 16. A high current, electronicswitch 72 is used in the Current Pulsing Module 66.

The Current Pulsing Controller 62 (FIG. 14) synthesises a cutting speedsignal, which may be derived from a tacho generator or encoder signalscorresponding to each axis of the machine, corresponding to the velocityof the profiling machine motion axes. In general, the cutting speedsignal is used to implement the control law which determines the arccurrent pulsing frequency, pulse duty, upper and lower current values.In this embodiment, the upper and lower current values are determined bythe nominal current commands 74, 76 (see FIG. 15) (or set points for thetwo constant current sources). The nominal current commands determinethe upper current value (I1+I2) and the lower current value I1, i.e.,the depth of pulsing is given by I2/(I1+I2). The current pulsing command78 is a square wave signal having required frequency and the pulse duty.The current pulsing command is generated by the Current PulsingController 62 and drives the switch 72 in the Current Pulsing Module 66.

In other embodiments, instead of using a cutting speed signal, the angleof the stream of molten metal from the kerf, the size of droplets ofejected molten metal, or the intensity or spectral pattern of lightemitted from the plasma arc jet and metal interface may be utilised. Theangle of the stream of molten metal and the size of droplets of moltenmetal, or the intensity or spectral pattern of light emitted from theplasma arc jet and metal interface can be derived on-line from images ofthe cut region obtained by a suitable optical sensor or sensors locatedbehind the cut plate or surface. The control algorithm may beimplemented in an analog or digital form. The control parameters willdepend on the type of material being cut and the thickness of thematerial.

A new process is described for dynamically controlling the plasma arccutting process for the purpose of ensuring high cut quality for a widerange of cutting speeds. The process maintains a balance between theamount of energy per unit length of cut delivered to a workpiece to meltthe metal and the optimal momentum of the plasma jet needed to removethe molten metal from the workpiece. The balance is maintained bypulsing the arc current in a controller manner. The parameters of thearc current pulsing are dynamically adjusted in response to some cuttingprocess variables. In the examples described above, the pulse duty ofthe arc current pulsing is controlled on the basis of the cutting speed.

A process according to the invention facilitates:

1. Rapid control of the amount of energy per unit length of cutdelivered to the workpiece and almost constant momentum of the plasmajet.

2. Enhanced atomisation of the molten metal in the cutting front andtherefore more effective removal of the molten metal from the workpiece.

3. Improved, uniform and repeatable cut quality for a wide range ofcutting speeds.

4. Improved, uniform and repeatable cut quality along the part profile,and in particular near sharp corners, because of the control of thepower delivered to the workpiece linked to the control of the torchmotion.

The invention described herein is susceptible to variations,modifications and/or additions other than those specifically describedand it is to be understood that the invention includes all suchvariations, modifications and/or additions which fall within the scopeof the following claims.

What is claimed is:
 1. A process of cutting a material using a plasmatorch, including (i) supplying a plasma forming gas and an electricalcurrent to the plasma torch for generating a plasma arc jet, (ii) movingthe plasma torch relative to the material for the plasma arc jet to cutthe material, (iii) pulsing the current supplied to the torch to controlthe amount of electrical energy of the arc per unit length of cut, and(iv) varying the pulsing of the current to maintain the momentum of theplasma arc jet substantially constant, notwithstanding variations in theelectrical energy of the arc.
 2. A process as claimed in claim 1 whereinthe pulsing to control the amount of electrical energy of the arc iscontrollably varied in dependence on a cutting process variable.
 3. Aprocess as claimed in claim 2 wherein the cutting process variable isone of: the angle of a stream of molten material ejected from the cut,the size of droplets of molten material ejected from the material, theintensity or spectral pattern of light emitted from the plasma arc jetand material interface, the arc voltage, and the cutting speed.
 4. Aprocess as claimed in claim 3 wherein the pulses are varied depending onthe cutting speed.
 5. A process as claimed in any one of claims 1 to 4wherein the pulsing of the current is varied by varying the pulsingfrequency.
 6. A process as claimed in claim 5 wherein the pulsingfrequency is controller such that the period of the arc current pulsingis of the order of the time constant of the pressure response of anozzle of the torch through which the plasma forming gas flows to a steparc current change.
 7. A process as claimed in claim 6 wherein thepulsing frequency is varied between about 150 Hz to 600 Hz.
 8. A processas claimed in any one of claims 1 to 7 wherein the pulses are varied byadditionally varying any one or more of: the pulse duty, upper currentvalue and depth of pulsing.
 9. A process as claimed in claim 8 whereinthe pulse duty is varied.
 10. Plasma arc cutting apparatus including: aplasma torch for providing a plasma arc jet for cutting a material, apower supply for supplying a current to the torch for generating theplasma arc jet, means for pulsing the current to the torch, wherein thepulsing means is operable to variably pulse the arc current such thatthe momentum of the plasma arc jet is maintained substantially constantwhilst the electrical energy of the arc is varied.
 11. Apparatus asclaimed in claim 10 including means for measuring a cutting processvariable to supply a signal to the pulsing means to variably pulse thearc current in dependence on the cutting process variable.
 12. Apparatusas claimed in claim 11 wherein the means for measuring a cutting processvariable measures the cutting speed.
 13. Apparatus as claimed in claim11 wherein the means for measuring a cutting process variable measuresthe angle of a stream of molten material ejected from a material beingcut.
 14. Apparatus as claimed in claim 11 wherein the means formeasuring a cutting process variable measures the size of droplets ofmolten material ejected from a material being cut.
 15. Apparatus asclaimed in claim 11 wherein the means for measuring a cutting processvariable measures the intensity or spectral pattern of the light emittedfrom the plasma arc jet and material interface.
 16. Apparatus as claimedin any one of claims 10 to 15 wherein the means for pulsing the currentto the torch is operable to vary the pulsing frequency.
 17. Apparatus asclaimed in claim 16 wherein the means for pulsing is operable toadditionally vary any one or more of the pulse duty, upper current valueand depth of pulsing.